Conjugated molecules based on quinoidal unit have attracted much attention as organic semiconductors for application in OFETs due to its structural features with high planarity and rigidity. Their planar conjugated structures induce extension of delocalization of π-electrons along the molecular backbone and strong intermolecular interaction via π-π stacking; therefore, quinoidal molecules manifested excellent charge transport properties. Recently quinoidal conjugated molecules end-capped by the isatin have been reported owing to easy and facile synthesis for forming the quinoidal structure and availability to control the solubility via tuning alkyl chains at the N-lactam position of isatin. However, isatin terminated quinoidal molecules have the possibility to exist geometrical isomers due to be connected to a double bond linkage between aromatic rings. Thereby, these molecules have difficulty purifying unavoidably produced isomers as well as understanding the exact molecular packing model and structure-property relationship.
In this work, the concept of incorporating short side chain into the quinoidal core unit was introduced to obtain the isomer-free isatin terminated quinoidal molecules, quinoidal bis-3,4-dimethoxythiophene (QuMeOBT) and quinoidal bis-ethylenedioxythiophene (QuEDOBT). These quinoidal molecules were clearly identified as isomer-free by NMR analysis. Their thermal, optical, electrochemical properties were characterized. Interestingly, a QuMeOBT single crystal could be obtained by the solvent evaporation method, and the crystal structure was determined via single-crystal X-ray diffraction. To investigate intrinsic charge carrier transport properties of the QuMeOBT, single-crystal field-effect transistors (SCFET) were fabricated in bottom-gate/top-contact configurations.